Threads cut incorrectly on new radiator?

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Hi, I have fitted quite a few radiators over the years but have a new problem. I have bought a new column radiator - fairly cheap and no doubt imported. I've tried screwing in radiator tails dry, ie no tape - just to make sure they fit - and only get half way in before becoming really tight. I can force them further with a spanner, but on removal I can see the thread of the tails is being damaged. I've tried a few different tails and they are all the same. I am concerned that ptfe will never get a seal due to the tight metal to metal contact. Is there a way around this? Or should I return the radiator for a refund?
Thanks in advance for any help anyone can offer.
 
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Aren’t they supposed to get tight halfway in? Seems normal to me. Just tape them and screw them in - I’m sure everything will be fine.
 
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What is the length of thread ,of the tail, that goes into the rad ? And what is the length of the thread in the radiator ?
 
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Mottie, thanks for the reassurance, I probably will give it a go. I won't be ready to install it for a few weeks, so I just wanted to identify whether I had a problem or not. Terry, the tail has a thread approx 13mm long, and as best as I can measure the thread in the radiator is around 12mm. I thought tails generally were quite a loose fit, hence needing 10-20 wraps of ptfe to take up the gap and make a seal, however as you can probably tell I'm no expert on this!
 
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If most of the length of the tails 13 mm thread goes into the rad then it should be fine.
 
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if you do recap it, cover the tap in thick grease.
It will collect the scarf so it comes out with the tap
 
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If they're the older type tails (with a flange on the valve end) then long as you're getting 5 or 6 turns in it'll be fine, bit of Boss white or PTFE if you want to be sure. I got fooled the other way round (with modern tails that you put a compression fitting on)- couldn't believe how much PTFE I had to put on them before they sealed!
 
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Thanks guys, I do like the idea of running a tap through it. Is this the right one? RS PRO HSS 1/2 in BSPF Straight Flute Tap Tap Set | RS Components (rs-online.com) - not sure what the F is in BSPF. Do you just turn it in with a spanner?
BSP -Fitting. With 14tpi it is the right one but very expensive just for cleaning an existing thread imho. You can use a spanner instead of a tap wrench but keep the tap true to the hole and it shouldnt need a great deal of force.
 
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Thanks for these comments. I agree it's expensive - it's a 2-piece set with a "plug" and a "taper". Doesn't mean a lot to me so I'll investigate further before buying anything. 'oldbutnotdead', that has also been my previous experience - I have modern tails with compression fittings for the valve, so was expecting to use a lot of ptfe. I've just tried 10 turns of ptfe and I can screw them most of the way in - but still with that unsettling metal on metal grinding which I worry is destroying the effectiveness of the ptfe.
 
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The "Plug" tap cuts a thread to the bottom of a blind hole so is of no use to you here
 
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